Ok, spoiler: As far to the right as you can go in the dungeon, past the boxes you must climb to escape a pacing bull, in the passage that leads to the black treasure box, there is a shelf, and under it is a basket. This is what you seek.
I think I know which black box you're talking about but I don't know how to enter that door area. Tried all doors but none leads there x.x
ok nvm figured it out thanks